There is a growing consensus in forest-based industries that felling heads have become too large, complex, and costly. Head weights in the 4000-6000# range are the norm. They require large carriers to provide the stability and power needed thus amplifying the weight, cost, and complexity aspects. In addition to the negative economic aspects of the foregoing, environmentalism demands that we reduce, to the greatest extent possible, the intrusive nature of harvesting operations on the forest environment.
Existing carriers take the felling heads to every tree if they are directly mounted on the carrier. If a boom is employed, they can reach several trees from one carrier position. These booms tend to be short and the carriers heavy in order to provide the necessary stability for the heavy felling heads. In either case a lot of traffic over the forest floor is necessary. This means a lot of soil degradation and damage to young tree growth.
These negative effects all stem from felling head weights which are reduced in proportion to reduction of those weights.
The types of severing devices in use and the grapple mechanism required to permit accumulation of cut trees are the major determining factors in felling head weights. In recent years there has been a strong swing to different types of saw severing devices. Many of these have added greatly to the weight and there is a growing interest in returning to lighter shearing devices - particularly for small tree operations such as thinning and harvesting the Boreal Forest where cut quality is not so important.
Accumulating capability so improves the performance of a felling head that it is mandatory on all heads. Grapple systems, which permit accumulation on felling heads, usually comprise 3 or 4 arms. These may be a mixture of fixed and/or pivoted. The pivoted arms may be one-piece or two-piece articulated (spring loaded or fully powered). In operation one or two arms (holding arms) hold the trees already cut and the remaining one or two arms (grappling arms) are used to encircle the next tree and move it into the accumulation once it has been cut. In general the holding arms are of the articulated type as is illustrated in U.S. Patents Kurelek 4,491,163- Hamilton et al 4,690,185 and MacLennan et al 5,004,026. Articulation permits withdrawal of the holding arms from between the accumulated trees and subsequent tree being moved into the accumulation by the grappling arms. The grappling arms may be one-piece or the two-piece articulated type.
My U.S. Pat. No. 3,920,057 discloses a shear type felling head. The shears are supported by inclined arms and two one-piece grapple arms are disclosed. The shears are powered by a single cylinder and the grapples preferably by separate cylinders. There are abutments above and below the grapples to locate the head on the tree and stabilize it once it has been cut. There are no special provisions for accumulation and if it is attempted the cut tree being held will fall vertically, when the grapples are opened to receive the next tree, and be cut again.